Maximum Compression Ratio On Olds 455 - What's Your Problem?

Squeeze ItDaniel Stafford; Nicholasville, KY: I've looked all over the Internet and asked the opinion of everybody I can think of, but I can never seem to get a definitive answer to my question. What is the maximum compression ratio I can run on my 0.060-over Olds 455 on premium pump gas? This engine will reside in my '68 4-4-2 convertible and is sporting Edelbrock aluminum heads with a Mondello super competition port job, Probe aluminum pistons, and an Offy intake. Is there a straightforward answer or are there some variables that should be taken into consideration? I'm sure the setup and type of ignition system play a big part in determining the maximum compression ratio I can run without worries of detonation, overheating, and so on. Also, I have yet to decide what type of cam to install, as I'm sure cam type and timing play a role in this equation. The more power, the better. Does 10.0:1 or even 10.5:1 sound outlandish?

2/12Olds engines make great torque. With a cam around 236 degrees at 0.050 and a set of Edelbrock heads, you could expect around 450 hp and 500 lb-ft. Peak horsepower will be around 5,800 rpm. This is the Olds 455 we built in the Oct. '10 issue.

Jeff Smith: To answer the question directly, Daniel, a static compression ratio of 10.5:1 with a performance camshaft of around 230 degrees at 0.050 would be a great combination. At that compression, don't get too aggressive with the timing on pump gas-around 34 degrees should be about right. While everyone talks about horsepower, keep in mind that some engines are more torque oriented. The Olds engines were intended to make mega torque but not necessarily as much horsepower mainly because you don't spin these engines as high as others. This is due to many factors, not the least of which is that the 455 Olds uses a monstrously heavy rotating assembly that includes a 3.00-inch main journal crankshaft. Basically, these engines are intended to run at 5,500 rpm and below. You should not treat a 455 Olds like a small-block Chevy unless you spend big dollars for quality rotating assembly parts. In our Oct. '10 issue, we built a 455 Olds with Edelbrock heads with 10:1 compression and a mild 224/230 cam at 0.050-inch tappet lift and 0.485/0.490-inch lift that made 511 lb-ft at 3,700 and 443 hp at 5,100. I didn't check the cranking pressure, but my guess is that with 10:1 static compression, it will be around 195 psi. Let's dive a little deeper into the relationship of cam timing to compression.

As you have guessed, there are many variables that play into the selection of compression ratio and cam timing. I'll narrow this down to a few essential points. Cam timing has a very distinct effect on engine performance. A long-duration camshaft is intended to increase the amount of time the intake valve is open to help engine breathing at higher engine speeds. A longer-duration cam also means the intake closing (IC) point occurs much later than a shorter-duration intake lobe. If you think about how a four-stroke engine operates, the cylinder cannot begin to compress the air and fuel until after the intake valve closes.

Static compression ratio is defined as the ratio of the cylinder volume with the piston at the bottom of its travel (bottom dead center or BDC) compared with the volume of the cylinder with the piston at the top of its stroke (top dead center or TDC). Static compression ratio does not take into account when the intake valve opens or closes. So 10 times more volume at BDC than at TDC creates a 10:1 static compression ratio. Notice the use of the term static. Dynamically, the engine sees a much different situation. Since the cylinder cannot begin to create pressure until the intake valve closes, with a later-closing intake, less air and fuel will be captured in the cylinder, especially at low engine speeds. Plus, long-duration camshafts routinely allow the piston to push a certain amount of air (accompanied by a small amount of residual exhaust gas because of the earlier opening intake valve) back into the intake manifold at idle and low speeds. This residual exhaust gas is one reason an engine with a big cam idles so roughly.

When there is less air compressed at slow engine speeds, the engine torque is not as high as it could be. A short-duration camshaft makes more low-rpm power than a long-duration cam because the intake valve closes sooner, allowing the cylinder to squeeze more air and fuel. At high speeds, the longer-duration cam allows the engine more time to ingest air and fuel to make more power. All this means you need more static compression with a longer-duration camshaft to compensate for the later-closing intake valve. This can be evaluated by using cranking compression. A long-duration cam with more static compression will still deliver decent cranking compression of 180 to 190 psi, while a long-duration cam with low static compression will lose cranking pressure down to perhaps as little as 160 psi-depending on the size of the cam. A cranking cylinder pressure maximum of 200 to maybe 210 psi is the most you can expect and still avoid detonation on 91- to 93-octane pump gas. A further advantage of more compression is it will dramatically improve throttle response and crispness on the street, not to mention its minor contribution to better fuel mileage.

As far as your question about ignition timing, a good place to start is 34 degrees of total mechanical advance. If the engine rattles on good gas, just retard the timing 2 degrees until the detonation is gone. The best way to evaluate maximum timing is at the dragstrip by watching the trap speed. If you add timing and the trap speed increases, keep going until the trap speed falls off and then return to your best mph tune.

Hot AirDave Visaggi; Scarsdale, NY: I have a '67 Pontiac LeMans with a Chevy 330hp 350ci crate engine. It has Vortec heads and a Holley 650 vacuum-secondary carb. It also has a serpentine beltdrive, which turns a reverse-spin water pump meant for '84 to '91 Corvettes. The car has an OEM-style radiator with top and bottom tanks. It also has aftermarket A/C installed. It's a daily driver, and I am struggling with a cooling system problem.

3/12The best way to maximize airflow through the radiator with an engine-driven fan is to use a shroud that completely covers the radiator core. This ensures that 100 percent of the radiator contributes to reducing engine temperature. Then try to maintain a tip clearance of 3/4 inch between the end of the fan blade and the shroud opening.

My car runs hot in traffic on 95-degree days. I would prefer to cool it with only a radiator, a beltdriven fan, and a shroud. I like to avoid electrical parts because when they quit, there is usually no warning. I would also like to avoid cutting up things to install a big radiator. Is there any way to size a radiator to the cooling load to avoid trial and error? I would like to stick the next size radiator in the car but would hate to waste the money if I then find I have to go bigger and start cutting. Also, what is the appropriate thermostat for that engine? I currently have a 180. I would prefer a 195 because the car is just a bit stinky at idle, and that appears to go away when it is running hot. My '97 Suburban ran all day long at 210, and I figure my combustion efficiency and thus fuel economy and power would improve with higher water temp. This is all about enjoying the car from behind the windshield.

Jeff Smith: Dave, let's see if we can help make your car more fun to drive. To start with, it appears the stock vertical flow radiator for a '67 Tempest is 155/8 inches tall and 243/4 inches wide. This should be more than sufficient for a small-block Chevy (assuming it's in good condition), so a larger radiator probably isn't necessary. I noticed a couple of clues in your description. First, your car suffers from the classic low-speed overheating problem. You don't say exactly how hot the engine gets, but from the description of your Suburban running at 210 degrees F, I'll assume this is not "hot," which is good. Next, you say you are running a serpentine beltdrive package using a reverse-rotation water pump. It also sounds like you are currently using an engine-driven mechanical fan. My guess is that the engine-driven fan you are using is likely intended for the more common V-belt accessory drive systems. This works very well as long as the fan is spinning in the proper direction. However, your reverse-rotation water pump spins in the opposite direction. Spinning a normal-direction fan backward will still move air, but the efficiency is greatly reduced. What you need is a reverse-direction fan.

Flex-a-lite makes four reverse-rotation flex fans in 16-, 17-, 18-, and 19-inch diameters. With the stock-width radiator for a '67 Pontiac, you can easily stuff in an 18-inch-diameter fan (PN 1518, $53.95, SummitRacing.com), but you will need a good shroud to make it work really well. A larger-diameter fan pulls more air because the blade tip speed is faster for any given engine speed. This higher tip speed will generate a greater pressure drop behind the radiator. Virtually as critical as the fan is a properly designed shroud. The shroud's purpose is to pull air through the entire surface of the radiator core. Make sure the fan is never closer than 1 inch from the face of the radiator, and you should also have at least 1/2 inch between the trailing edge of the fan tips and the accessory drive. You can also increase airflow through the radiator by sealing all the areas ahead of the radiator core support. This will force all the air entering the grille to travel through the radiator rather than around it.

If your radiator is marginal, you have a choice between aluminum and brass/copper. Aluminum is lighter, generally offers a larger capacity, and will do an excellent job of cooling. If you're on a budget, look for a brass/copper unit with a high-density core. The one company we know of that sells these is U.S. Radiator. Less expensive brass/copper radiators are often fitted with low-density cores. To compare them, measure the space between the coolant tubes. High-density cores will feature less distance between the tubes because there are more tubes within a given core size. Then you can run a 195-degree thermostat that will maintain that higher temperature. Remember that a thermostat's only job is to maintain minimum engine temperature. Frankly, if the engine is stinky at lower engine temperatures, very little will change when it completely warms up (unless we're dealing with an electric choke that's set too rich). I would suggest addressing the problem with a careful adjustment of the idle-mixture screws at the lean limit along with checking the initial timing. Run a minimum of 10 degrees initial timing, and if the engine doesn't rattle on hard acceleration, bump that up to around 12 to 14 degrees initial with a total of around 36 degrees mechanical advance. These changes and a functioning vacuum-advance curve will really help move you toward your goal of a cool-running vehicle that gets decent mileage. And that will be much more fun to drive.

Puking OilGerald Lum; Rolling Hills Estates, CA: I'm running a Melling M55HV oil pump in my 427ci small-block. With Castrol 10W-40 GTX oil, it gets about 50 to 55 psi at a steady 2,700 rpm. When I autocross the car, I get some oil barfing out of my valve cover filters. The oil is coming out of the outside valve covers at high rpm on a skidpad at 1 g. It even blew out my dipstick. I never had that problem with my 355ci iron-block engine at much higher engine speeds with the standard Chevy oil pump using the same valve covers. Should I just replace the high-volume oil pump with the standard Chevy one? Would that solve my problem? I think it's because the Melling M55HV may be putting out too high a volume of oil for my 427 at high rpm on a skidpad and flooding the outside valve cover with too much oil. The oil has nowhere to go except into the breathers on the outside valve covers. I know some racers use a high-neck filter or the Moroso setup in which tubes from both valve covers connect to a single filter to solve that problem. I'm wondering if the HV pump could be flooding the valve covers.

4/12Leakdown testing involves pressurizing the cylinder and measuring the amount of leakage expressed as a percentage. Good numbers will be 10 percent or less. Our LS engine measured roughly 20 percent, but this was on a cold engine. At normal temperature with oil sealing the rings, the number dropped to an acceptable 9 percent.

Jeff Smith: It is true that the typical small-block Chevy will tend to push quite a bit of oil into the valve covers under extended rpm. By your description, it appears that the addition of a high-volume (HV) pump will tend to move more oil up into the valve covers, which could easily result in a portion of that oil taking residence inside the breather, creating the barfing (a great adjective by the way) that ends up spraying all over your valve covers and headers. The Moroso solution is really nothing more than a length of tubing (I've used radiator hose) to connect the two valve covers with either a single or multiple breathers. This places the breathers higher, which will prevent the oil from escaping. It also helps to include a vapor separator in either the valve cover or inside the breather stands above the valve covers. These separators will prevent oil from reaching the top of the breathers. At first, it appeared that the high-volume pump was likely the culprit. But engine builder Kenny Duttweiler says most NASCAR engines run with large volumes of oil in the valve covers to help cool the valvesprings. He didn't see anything wrong with retaining the high-volume pump and the amount of oil in the covers. As long as you don't have an oil starvation problem in the corners that would cause the oil pressure to drop (because there's more oil in the valve covers than there is in the pan), it appears that creating a decent breather system would be the best solution. You are correct that changing to a standard-volume pump will reduce the amount of oil pumped to the top of the engine.

However, your comment about the dipstick pushed out is cause for concern about excess crankcase pressure (blow-by). Perhaps the engine does not have sufficient venting. This would cause the dipstick to push out from excessive pressure. It's worth mentioning that excessive crankcase pressure is generally linked to poor ring seal. Once pressure leaks past the rings, it ends up in the crankcase and the rest of the engine and tries to push its way out. It would be best to identify if there are specific cylinders with problems or if all eight holes share a common affliction. The first test is to perform a cylinder leakdown test. A leakdown tester pressurizes the cylinder (with both valves closed) and is fitted with a gauge that will indicate a percentage of leakage. A relatively new engine at operating temperature should deliver a leakage of less than 10 percent. Despite all the claims you will read on the Internet, 8 to 10 percent on a new engine is not all that unusual. What is bad is if you see 30 or 40 percent in one or more cylinders. These numbers are based on testing with the engine at normal operating temperatures. If you test the engine cold, the leakage will be significantly higher. It's possible that all eight cylinders are experiencing difficulties.

I'm assuming this is a relatively new engine, so it's possible that the engine builder mistakenly installed a top compression ring upside down, which would instantly create this situation. If all the rings are inverted, that is the root of the blow-by problem. While this is an easy fix (I would suggest a new set of rings rather than reusing the old rings), it obviously means you have to remove the pistons to repair the problem. At the very least, I would suggest a thorough cleaning of the cylinder walls.

Assuming that the leakdown is higher than acceptable, the solution is time consuming but not difficult to repair. Jeff Latimer at Jim Grubbs Motorsports (JGM) gave me some great insight into aluminum block engines. He said JGM routinely hones all aluminum blocks with torque plates on both sides of the engines along with fully torqued main caps to simulate the stresses these engines experience. He also told me that honing an engine in which only one torque plate was used causes the stones to skip and you will see major shadows on the cylinder wall that indicate the cylinder was not round. This occurs because the cylinder walls have moved as a result of block distortion. Latimer said this is extremely common with aluminum LS engines. Many machine shops do not use dual torque plates because it doubles the investment they must make in tooling for each engine. I'd suggest asking the machine shop its exact procedure for torque plate honing. Let them tell you what they do-don't ask if they use two plates. If you do not know how the machine work was accomplished and the leakage numbers are high, then it would benefit you to completely disassemble the engine and have the cylinders honed with this procedure. Latimer also mentioned that JGM uses the same head gaskets that will be used in the engine and the same fasteners (studs or bolts) if possible. This helps to ensure that the engine is stressed as closely as possible to the way it will run in the car. We've heard of shops that will even bolt engine plates and motor mounts to the block.

More InfoJim Grubbs Motorsports (JGM)
Valencia, CA
661/257-0101

Lobe-Separation AngleRon Cotter; via CarCraft.com: First and foremost, I love the magazine. My dad and I have been reading it as long as I can remember. I have a question regarding lobe separation. I was recently talking to a respected engine builder about an engine a friend of mine built that wasn't pulling enough vacuum. The gentleman suggested that my friend find a cam ground with a wider lobe separation to help with the vacuum. I understand the valves wouldn't overlap as much with the wider separation creating more vacuum, but I'm wondering what you sacrifice in the long run. Thanks, and keep up the great work with the magazine.

Jeff Smith: Your timing this month was impeccable, Ron, since this column earlier touched on lobe-separation angle (LSA). You are correct that a smaller (tighter) angle between the intake and exhaust lobes increases the overlap between when the exhaust lobe closes and the intake lobe opens. This creates an overlap when both valves are open in the combustion chamber. The triangular shape of overlap in cam degrees can be easily seen in the accompanying Comp Cams illustration. It allows a crosstalk between the exhaust and induction systems that tends to reduce intake manifold vacuum at idle and low engine speeds.

Overlap is generally something the new-car manufacturers tend to avoid because it drastically lowers idle quality. But the advantage to overlap is it allows the engine to breathe more efficiently at higher engine speeds, which dramatically improves power. This doesn't happen just at peak horsepower. Dyno testing has shown that just tightening the lobe-separation angle from 114 to 110 degrees with a mild street engine can be worth as much as 10 to 20 lb-ft in the midrange of around 4,000 rpm, and the engine will even see a torque increase as far down as 3,000 rpm under wide-open throttle. Pulling the lobe even tighter to around a 108-degree lobe-separation angle will improve the power even more, especially in the midrange.

But before we set this concept in stone, we have to also look at the actual number of degrees of duration and overlap. If you look at longer-duration camshafts, you may notice that these cams often use a wider lobe-separation angle of 112 to perhaps 114 degrees. This is because a longer-duration camshaft (280 degrees at 0.050 for example) with a tight lobe-separation angle will physically have far more actual degrees of overlap than a smaller camshaft of say 230 degrees at 0.050. So you have to be careful when referring to LSA angles because the physical amount of actual overlap will change with the size of the camshaft. Based on all this information, you can see why many camshaft companies prefer to use a 110-degree lobe-separation angle because it's a great compromise between idle quality and increased power. It is also why dedicated, normally aspirated, drag race engines tend to idle with a radical sound-they've tuned the LSA for best overall power. The problem with a tight LSA on a street engine is that it tends to be lazy and requires a rich air/fuel mixture to idle properly with a carburetor and an automatic transmission if the converter is a little on the tight side. This is what we've run into on our Lester Scruggs 404ci LS engine with the big camshaft with lots of overlap at idle. To make the engine run in gear, we have to tune the carb to a rich 12:1 air/fuel ratio in Neutral or Park so it will idle in gear without dying. That's part of the sacrifice of a camshaft with lots of overlap.

TV TricksBill Irwin; Thayer, IA: I'm building my own version of your Lester Scruggs 404ci LS engine that ran in the Feb. '10 issue. I want to put it in a street rod, but instead of a carburetor, I'm planning on running the production EFI. It appears that you have had good luck with the 200-4R overdrive automatic in your Chevelle, and I was thinking that would be a better trans to run instead of the 4L60E. How would you adapt the TV cable for the 200-4R trans to an LS-style throttle body?

6/12The Bow Tie Overdrives TV cable adapter for LS-series engines allows you to use either a 200-4R or the 700-R4 TV cable-style transmission with an EFI-controlled LS engine.

Jeff Smith: Bill, that's a great question. My search led to me to Bow Tie Overdrives (BTO), which makes an adapter that allows you to connect the throttle valve (TV) cable directly to a typical LS mechanical throttle. One distinct advantage to the BTO adapter is that it is adjustable. The adapter allows you to adjust the tension on the TV cable, which will alter the shift timing and firmness. The TV cable replaced the function of the older vacuum modulator valve that uses a vacuum hose connection between the transmission and the engine to indicate load. With the TV cable, as soon as the throttle is opened, the cable moves the throttle valve inside the transmission, raising the line pressure. This affects the shift points and shift firmness. The key is to adjust the movement of the TV cable until you achieve the desired shift point changes. The BTO website offers excellent instructions on how to mount the adapter and adjust it, and the company also offers an outstanding description of how the stock TV system works (called TV Made EZ) accompanied by excellent graphics. The three-part description illustrates the stock system's shortcomings and what BTO describes as the "short spring syndrome" that causes problems with TV tuning. To this end, the company recommends using a specific BTO spring, which must be installed in the valvebody. It's a simple swap but requires dropping the transmission pan. The LS1 cable and throttle body adapter is PN TVEZLS1BR ($159.95). This cable requires a specific separate cable mount bracket (there are four different ones depending on your application) that is an additional $10. I would highly recommend you read the entire three-part series to fully understand how the throttle valve system works, which will make adjusting the system much easier and far less confusing.

While investigating the BTO website, I ran across a great adjustable trans mount that works really well with LS1 engine swaps. The mount has a slide adjustment feature that allows it to move fore or aft a total of 2.5 inches, which is more than enough to accommodate the various engine mount variables present in LS1 engine swaps. This adjustable mount may prevent the need to either drill new holes in the framerails for moving or having to make or purchase a custom transmission crossmember. This is a very slick little adapter for any car crafter considering an LS engine swap. The adapter part number is SLDEMOUNT and sells for $110.

7/12If you are swapping an LS-style engine into an older car, BTO's adjustable trans mount may save you some effort. We've identified at least four different LS engine mount positions depending on the aftermarket manufacturer. Most of these mounts will also require moving the transmission mount position. The adjustable BTO mount solves that issue with almost 5 inches of slide movement.

Rat ChroniclesDoug Razze; Franklinville, NJ: I bought my first 454! Little did I know, nobody knows anything about Gen V engines. Can you help me bring this boat anchor back to life on a budget? Some say I can use old 454 heads, and some say use marine heads. Please help! This is a good running motor but not worthy of my '70 Camaro.

8/12This is a GEN VI L29 454 that a friend recently purchased for a mere $220 along with a 4L80E automatic transmission. You can quickly tell a Gen VI by its six-bolt timing cover.

Jeff Smith: The 454 big-block Chevy went through a series of significant changes in the '90s that directly affect parts interchangeability. We'll hit the high points and then clue you in on some affordable changes that will help this aging Rat make some power. The first-generation big-block from '65 to '90 is commonly referred to as the Mark IV, and all the parts interchange. GM's '91 revision, known as the Gen V, made drastic changes. Starting with a one-piece rear main seal, the new Rat retained the stock head bolt pattern, but changes to the upper coolant holes in the block made converting to older Mk IV heads not impossible, but certainly problematic. The Gen V also eliminated the mechanical fuel pump boss, moved the main oil gallery alongside the camshaft, and enlarged the diameter of the freeze plugs. While the Gen V retained a flat-tappet hydraulic camshaft, the valvetrain was changed to a net lash system that eliminated the rocker arm stud, replacing it with bolts that did not allow valve lash adjustment. Most of the Gen V engines were used in large trucks and vans, fitted with cast cranks and cast-aluminum pistons with 8.0:1 compression and 118cc chambers. Then in 1996, GM revised the big-block yet again, giving it a hydraulic roller cam and multipoint fuel injection. The block coolant passages were changed slightly, making a conversion to Mk IV heads easier. The best way to tell a Gen VI engine is by its six-bolt cast-aluminum timing cover and large multipoint EFI manifold. Finally, GM ruined Rat interchangeability by converting to an 8.1L (496ci) version that suffered from so many revisions (including a completely different head bolt pattern) that no parts interchange, effectively killing this Rat as a performance platform.

It sounds like you're on a budget, so the easiest way to pump a little iron into this Rat is with a cam and breathing. We'll assume the short-block is in decent shape. While many enthusiasts still claim you can only make power with a set of rectangle-port heads, the reality is the factory oval-port castings are perfectly suited for street performance. If you are willing to spend a little money, the best move is to remove the heads and have your machine shop mill the net valve lash stands and drill and tap the original 3/8-inch boltholes for 7/16-inch head studs and guideplates. That will allow you to run a decent performance camshaft and good aftermarket rocker arms. You might also consider milling the heads to improve the static compression. Milling the chambers to 110 cc will bump the lame 8.0:1 compression up to 8.5:1. Also make sure the valveguides are in good shape. Even if the valves are in decent shape, consider increasing the valve size from the stock oval-port 2.06/1.72 inches up to at least the stock rectangle-port head 2.19/1.88-inch valve dimensions.

Because the compression is so low, you have to be conservative with cam timing using a mild hydraulic flat-tappet. The Lunati Bare Bones big-block Chevy version specs out at 214/224 with 0.501/ 0.527-inch lift combining the cam and a set of lifters for a mere $115.95 from Summit Racing. Top this off with an Edelbrock Performer RPM dual-plane intake (PN 7161, $219.95, Summit Racing), a 750-cfm Holley carburetor, and a set of 13/4-inch headers and you're on your way. This package should make about 450 hp and might push upwards of 500 lb-ft.

Cop Car Coilovers?
In this month's Speed Shop, we mentioned the coilovers on our Crown Victoria. Here is a picture. They are QA1 shocks and springs modified by Naake Suspension to fit in the stock attaching point in the lower control arm of '03 and '04 Mercury Marauders and '03-and-later Crown Victorias. Go to Naake.com to see the kit.

9/12

CC QuickesVintage Draw-Through
We recently took a trip to the Bonneville Salt Flats and spotted this engine in a stretched Model A. It is a small-block Chevy with a crank-driven Potvin supercharger being fed by eight carbs. We didn't hear it run, so it might just be art.

10/12

It Feels Faster, I Swear
In other cop car news, we are planning some 4.6L, two-valve, bolt-on articles, so here's our car at Racers Edge Tuning in Downey, California, for baseline numbers. The bad news: 208 hp at the wheels. The good news: It can only get better.

11/12

Mile-Long Timing Chains
Fans of the DOHC 4.6, don't fret. We are also working on some 4V build articles using this '95 Lincoln Mk. VIII long block we bought from Midwest Mustang in Lawrence, Kansas. We just dropped it off at QMP Racing Engines in Chatsworth for inspection and machining. Look for articles starting in a few months.